Medicare covers only 59% of the cost of health care for seniors–and retirees can expect to pay an even larger share in the future. In fact, a couple age 65 might need $387,000 saved in order to be confident of covering their health care costs in retirement, not including outlays for long-term care.

Those are findings from a new report from the Employee Benefit Research Institute (EBRI). Based on the latest data available, from 2009, Medicare paid 59% of health care services for beneficiaries 65 and older. Individuals enrolled in Medicare paid 13% out-of-pocket, private insurance covered 14%, and the balance came from other sources such as Medicaid and VA benefits.

Going forward, EBRI predicted that “individuals can expect to pay a greater share of their costs out of pocket because of the combination of the underfunded financial condition of the Medicare program and cutbacks to employment-based retiree health programs.” To put numbers on how much seniors will need, EBRI used a Monte Carlo simulation model; it came up with estimates based on enrollment in original Medicare supplemented with a Plan F Medigap policy and a Medicare Part D plan for prescription drug coverage.

EBRI found that estimated health care costs for seniors depends on two factors. First, how much are people likely to spend on prescription drugs? Second, how confident do they want to be that they’ll  cover their health care costs?

For example, take a married couple in which both spouses are now 65. They are in good health, so they expect to use the median amount of prescription drugs in retirement, and they are satisfied with a 50% chance of meeting their lifelong health care costs. Such a couple should have $163,000 in savings, EBRI concluded.

On the other hand, suppose that same couple expected to be in the 90th percentile of prescription drug usage and also wanted a 90% chance of having enough savings. Such a couple would need savings of $387,000, according to EBRI. Even those numbers might be inadequate, if long-term care is needed, but seniors may trim the amounts needed for health care if they work past age 65 and continue to receive health care benefits as employees.